In communicating information bits from a transmitter to a receiver through a wireless channel, the received bits must be demodulated and decoded so that the transmitted information bits can be accurately recovered. For example, in a wireless system that communicates by time division multiple access (TDMA) bursts, the transmitted burst goes through a transmitter, the channel and a receiver. In making this trek, however, the burst acquires imperfections which, in turn, make decoding of the burst error prone.
One of these imperfections is delay spread. Delay spread is caused by the reception of a burst in more than one instant at the receiver. In most wireless systems, the main concern involving delay spread is the effect of receiving a burst at more than one time instant (e.g. a burst and its ghost), wherein the power of each is different from the other. This problem is similar to the problems associated with the appearance of ghosts in TV images, which are caused by the reception of the transmitted signal by the TV antenna at two different instances.
An illustrative view of the timing involved in receiving a burst and its ghost at two different instances is shown in FIG. 1. As shown, a burst 10 and its ghost 11 arrive at the receiver at two different time instants. That is, burst 10 arrives at time 12 and its ghost 11 arrives at a later time 13. The difference between times 12 and 13 is the delay spread, hereinafter referred to as delay spread 14. Thus, delay spread 14 is the measure of time between the reception of the burst 10 and its ghost 11 at the receiver. It is this delay spread 14 that can significantly affect the decoding of the information bits received.
In a wireless system, the decoding problems associated with delay spread become more drastic because such systems transmit the information bits over a wireless channel whose characteristics change with time. This results is a time-varying delay spread. That is, a delay spread that changes with time as the wireless channel changes with time. As a result, getting rid of this ghosting problem, i.e. delay spread, to recover information bits in a wireless system can be an arduous task, and thus may require a complex decoding mechanism in the receiver. Consequently, depending on the distance of the delay spread and the strength of the ghost signal received, the receiver may require extra circuitry to reduce the effect of the ghost signal on the decoding of the information bits received for each burst. For example, the receiver may need to utilize an equalizer to get rid of the ghost before attempting to accurately recover the information bits for each burst.
In a wireless environment, however, constant use of an equalizer may not be the best approach for proper decoding of the received information bits. In general, in a wireless system wherein the channel varies very drastically and very quickly, the constant use of an equalizer to recover the information bits can be detrimental to the costs associated with recovering the information bits at the receiver if the equalizer was not absolutely necessary for reducing the effect of a ghost during reception of the burst. More specifically, in a wireless system wherein the ghost is received very close in time to the reception of the original burst, then no equalization is needed to minimize the effect of ghost-images. Whereas, if the ghost has significant power compared to the burst, and if the delay spread between the burst and the ghost is sufficiently large, then an equalizer is necessary to provide accurate information recovery at the receiver. As a result, depending on the strength of the ghost signal and the size of the delay spread between the burst and its ghost, an equalizer may or may not be needed for correct information bit recovery. Thus, wireless systems that employ an equalizer to minimize the effects of a ghost signal at the receiver are not efficient during times when the ghost is weak and/or close, in time, to the burst. And, wireless systems that do not employ an equalizer are not efficient when the ghost is strong and far, in time, from the burst.